Fiber optic communication system offer many advantages, including high data transfer rates and substantial immunity to electromagnetic interference. However, it is generally expensive to lay fiber optic cable, especially in dense metropolitan areas. This has led to increasing demands for higher data rates on existing fibers, and attempts to send light down longer stretches of fiber. Technologies for increasing the data rate of fiber optic communication include Course Wave Division Multiplexing (CWDM) and Dense Wave Division Multiplexing (DWDM).
As data rates and distance increase, limiting factors in fiber optic communications come into play. These limiting factors include Composite Second Order (CSO) and Composite Triple Beat (CTB) distortions, Cross Phase Modulation (XPM), as well as Stimulated Raman Scattering (SRS), non-linear Erbium Doped Fiber Amplifier (EDFA) response, and optical dispersion.
Many of these limiting factors are functions of optical power and optical separation (the frequency separation between communication bands in the optical signal).
Cable television optical systems may employ a broadcast signal BC (a signal intended for everyone in a particular distribution area) and several narrowcast NC signals (signals with a more targeted audience than the broadcast signal). The broadcast and narrowcast signals may be multiplexed onto a single optical fiber. Signals may be concentrated in bands around 1550 nm and 1310 nm where signal attenuation and/or dispersion tend to be at a minimum. Optical transmission power may be increased to push the signals down longer fibers, and adequate frequency separation between NC bands and between the NC and BC bands may be difficult to maintain. Consequently, the various limiting factors mentioned above, as well as others, may increasingly come into play.